1. Field of the Invention
The present invention relates generally to a process for electrophoretically preparing an insulation wire.
2. Description of the Prior Art
Processes have heretofore been developed for preparing insulation wires by electrodeposition so that the insulation wires have been commercialized as magnet wires. Two types of varnishes have been employed for electrodeposition coating, namely a water soluble type varnish and a water dispersion type varnish. The water dispersion type varnish has been considered to be superior to the water soluble type from the viewpoint of its electrodeposition rate and the thickness of the film as a varnish for insulation.
In the preparation of electric wires, the productivity and economy are dependent upon the wire running speed. In the conventional dip coating method, a high wire running speed, such as, for example, higher than several hundreds m/min., has been attained. Accordingly, a high wire running speed, such as higher than 50 m/min., has been required for an electrodeposition coating method.
It has been well-known to apply a film-forming auxiliary agent (polar organic solvent and polyols) after electrodeposition takes place in an electrodeposition coating method using a water dispersion varnish. In the case of the wire running at a speed of less than 50 m/min., there is to is no washing with water and applying the film-forming auxiliary agent after the electrodeposition. However, in the case of the wire running at speeds higher than 50 m/min., it has been found to increase the amount of a varnish drawn out from an electrodeposition bath, except the electrodeposited resin. Accordingly, the film-forming auxiliary agent in the post-treatment bath has been contaminated.
FIG. 1 is a schematic view showing the steps of a conventional process for preparing an insulation wire by an electrodeposition. In FIG. 1, a wire 1 is shown running along a line in the direction indicated by an arrow, being drawn through an electrodeposition bath 2, a washing shower 3, a post-treatment bath 4, a roller wiper 5, an air wiper 6, a semi-curing furnace 7 and a final curing furnace 8. The running wire 1 is passed through the electrodeposition bath 2, wherein a water dispersion synthetic resin is electrodeposited thereon and the excess of such varnish drawn out therewith is washed off by the washing shower 3. The wire is then passed through a film-forming auxiliary agent in the post-treatment bath 4 and the excess film-forming auxiliary agent thereon is removed thereafter by the roller wiper 5 and the air wiper 6. The resin layer on the wire is then heated by passing the wire through the semicuring furnace 7 and the final curing furnace 8 to cure it.
Thus, in the electrodeposition coating method using the water dispersion synthetic resin varnish, it has been necessary to apply an organic solvent as the film-forming auxiliary agent after the electrodeposition. However, the disadvantages of contamination of the organic solvent by the varnish drawn off with the wire and an increase of the amount of the organic solvent and the like have been found to be dependent upon an increase of the wire running speed. These troubles have not been found to occur in the case of a conventional low speed operation.
Accordingly, it has been necessary to search for an improvement of the process for preparing an insulation wire by an electrodeposition. In order to prevent contamination with the varnish in the post-treatment bath, removal of the varnish by an air wiper and a roller wiper, as indicated, has been considered. However, the former operation has not been suitable for a mass production apparatus wherein many wires are run, because of the swing of the wire by air pressure, and the latter operation has not been satisfactory because the electrodeposited resin layer is soft and is deformed even by a slight pressure, as the water content of the resin layer is about 50 percent by weight. In order to prevent excess varnish on the wire, it has been considered to wash the wire with water after the electrodeposition. In the process for preparing an insulation wire by an electrodeposition, the resin layer has thus been washed with water after the electrodeposition at high wire running speed. However, in the case of high wire running speed, the water is carried into the post-treatment bath so as to deteriorate the effect of the film-forming auxiliary agent within a short time. Accordingly, it is necessary to frequently provide a fresh supply of the film forming auxiliary agent, and the resultant economical loss is remarkably high.
It has also been proposed to combine an electrodeposition coating process with a dip coating process so as to attain the demands. However, in the proposed process for overcoating a varnish immediately after the electrodeposition, a large amount of water is incorporated in the electrodeposited resin layer. Accordingly, the electrodeposited resin layer is cracked or spotted to thus provide an insulation wire having an uneven coating layer, and it has been hard to obtain an insulation wire having good appearance and good characteristics.
More specifically, when a resin is deposited on a wire by an electrodeposition, the film is not solidified. Accordingly, the electrodeposited resin layer is damaged when overcoating immediately after the electrodeposition and the overcoated layer is significantly contaminated with varnish drawn out from the bath 2. Further, a wiper die for controlling the thickness of the overcoat layer cannot be used, whereby different thicknesses and uneveneness of the layer are disadvantageously caused by the wire running at speeds higher than 10 m/min. The method of heating the electrodeposited resin layer to dry it before the overcoating step, so as to overcome these disadvantages, has been difficult to practically apply because a resin layer having a thickness required for insulation that is greater than 10.mu. is significantly cracked by drying it and a desirable film is thus not obtained by overcoating.